Light transmission through optical fibers has meanwhile changed many areas of life enormously. Telecommunications, process technology and spectroscopy, fiber-optic sensors and medical diagnostics via catheters should be mentioned here.
Another important area is laser technology, where fiber lasers are gradually replacing other types of laser. You can find examples of all these applications elsewhere in this journal.
A typical optical fiber consists of a core fiber surrounded by a cladding with a lower refractive index than the core. The light is guided in the core by total reflection.
Another property of the fibers is used in laser technology. If a quartz fiber core is doped with ions, it can amplify light and thus be used as an active laser medium.
The most common dopings with typical emission lines are:
Erbium (1550nm), Ytterbium (1030nm), Neodymium (1064nm) and Thullium (1500nm). Double-cladding fibers have established themselves for efficiently coupling the pump light from high-power diodes into a doped fiber.
Here the pump light runs in an additional cladding around the core along the fiber. In this way a long interaction distance for efficient pumping of the core fiber is achieved.
A conventional solid-state laser consists of a pump diode, laser crystal and mirror resonator. If the laser crystal is replaced by a doped fiber and the mirrors by Fiber Bragg Gratings (FBG) incorporated into the fiber, which selectively reflect the laser wavelength in the core fiber, the fiber laser oscillator is obtained. The advantages are:
- high stability, since the mirrors no longer need to be adjusted
- high gain due to the long interaction distance along the fiber. Up to 70% of the pump diode light will be in
transmit diffraction-limited laser light.
When people talk about fiber lasers today, they often mean a combination of seed lasers and several fiber laser amplifiers, so-called EDFAs or YDFAs.
These amplify the output signal of a laser oscillator in several stages. The achievable laser power is limited by the damage threshold of the power amplifier fiber. This is typically 10kW for singlemode fibers and 50kW for multimode fibers.
Because of the high gain in the fibers, feedback must be prevented. Faraday isolators are usually used for this.
Continuous wave fiber laser
From the user's point of view, we distinguish between high-power lasers for material processing (metal cutting, welding, ...) and low-power lasers for various applications, e.g. in telecommunications or in biotechnology.
In the high-power range, IPG was recently able to demonstrate a fiber laser with a beam power of 18 kW. The frequency-doubled fiber lasers from MBP Communications are becoming more and more interesting for applications in biotechnology, since many colors in the visible range are now available with sufficient power.
Several wavelengths in the visible range are often required in a targeted manner, eg for flow cytometry or fluorescence microscopy, in order to excite the various fluorescence markers.
In addition to classic diode lasers, fiber lasers are increasingly being used here, the output of which can be efficiently frequency-doubled via periodically poled crystals, so-called PPLNs. With this technology, the fiber lasers from MBP Communications now achieve significantly higher powers of 1-5W than corresponding diode lasers.
Pulsed fiber lasers and MOPAs
For pulsed operation, fiber lasers are mode-locked (50fsek-50psek), Q-switched (10-500nsek) or used as MOPA (Master Oscillator Power Amplifier). Compact, maintenance-free, mode-locked fiber lasers with 50-100fsek and 100mW can be found, for example, in our time-domain THz spectrometers.
Raydiance presented the first short-pulse fiber laser with 700fsek and 5W for industrial applications. In the nanosecond range for material processing, the MOPA designs are slowly catching on. Here the output of a seed laser (laser diode, solid-state laser or fiber laser) is highly amplified in several fiber amplifiers. This concept brings an invaluable advantage over classic Q-switched lasers. Their pulse duration is determined by the gain in the resonator and thus varies greatly with the repetition rate, typically between 10-300ns.
In the MOPA, the seed laser defines the pulse duration and the fiber amplifiers deliver the energy without changing the pulse duration. The seed laser can be a laser diode whose pulse duration is adjustable. But it can also be a cw diode coupled with a fast electro-optical switch according to the figure above. This cuts out variable pieces from the cw signal, which are then amplified in the fibers.
In this concept implemented by ESI-PyroPhotonics Lasers, the transmission of the modulator can also be controlled and different pulse trains can be generated, including multiple pulses with any delays in between.
The pulse train, once set, can then be called up via an external trigger and retains its shape regardless of the selected repetition rate, in the case of the Pyrophotonics laser up to 500kHz. This property is very valuable for the optimization of certain laser processes in material processing.
Limitations of fiber lasers
Three properties must be carefully considered in fiber laser design to ensure reliable operation:
- the damage threshold of the power amplifier
- the high ASE (attenuated spontaneous emission) due to the high gain
- non-linear effects in the fiber such as SRS (stimulated Raman scattering), SBS (stimulated Brillouin scattering) and SPM (self phase modulation).
The ASE can be controlled well with suitable measures. Non-linear effects increase significantly with peak pulse power and adversely affect pulsed applications